Climate change is being more keenly felt by the sea's cold-blooded creatures.

As the world's average temperatures creep higher, marine animals are far
more vulnerable to extinctions than their earthbound counterparts, according
to a new analysis of more than 400 cold-blooded species.

With fewer ways to seek refuge from warming, ocean-dwelling species are
disappearing from their habitats at twice the rate of those on land, notes
the research published Wednesday in the journal Nature.

The study, led by researchers from New Jersey's Rutgers University, is
the first to compare the impacts of higher temperatures in the ocean and
on land for a range of cold-blooded wildlife, from fish and mollusks to
lizards and dragonflies.

While previous research has
suggested warm-blooded animals are better at adapting to climate change
than cold-blooded ones, this study punctuates the special risk for sea
creatures. As the oceans continue to absorb heat trapped in the atmosphere
from carbon dioxide pollution,
bringing waters to their warmest point in decades,
undersea denizens don't have the luxury of ducking into a shady spot or
a burrow.

"Marine animals live in an environment that, historically, hasn't
changed temperature all that much," says Malin Pinsky,
an ecologist and evolutionary biologist at Rutgers who led the research.
"It's a bit like ocean animals are driving a narrow mountain road
with temperature cliffs on either side."

Narrow safety margins

The scientists calculated "thermal safety margins" for 88 marine
and 318 terrestrial species, determining how much warming they can tolerate
and how much exposure they have to those heat thresholds. The safety margins
were slimmest near the equator for ocean dwellers and near the midlatitudes
on land.

For many, the heat is already too much. At the warm edges of the marine
species' ranges, the study found, more than half had disappeared from historical
territory as a result of warming. The rate for these local extinctions
is twice that seen on land.

The narrow safety margins for tropical marine animals, such as colorful
damselfish and cardinalfish, average about 10 degrees Celsius. "That
sounds like a lot," Pinsky says, "but the key is that populations
actually go extinct long before they experience 10 degrees of warming."

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A California sea lion hunts for fish on a kelp paddy at Cortes Bank, a seamount off San Diego.

A diver waits in still water under the crashing pipeline of Oahu Island, Hawaii.

Photograph by Paul Nicklen, Nat Geo Image Collection

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A bottlenose dolphin in Hawaii plays with a plastic six pack holder.

Photograph by Flip Nicklin, Minden Pictures/Nat Geo Image Collection

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Humpback whales feast on fish in the warm waters of Monterey Bay, California.

Picture by Paul Nicklen, Nat Geo Image Collection

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California's Highway 1 winds through Sonoma Coast State Park.

Photograph by Leah Nash, Nat Geo Image Collection

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Even just a degree or half-degree boost, he adds, can lead to trouble
finding food, reproducing, and other devastating effects. While some species
will be able to migrate to new territory, others—coral and sea anemones,
for example—can't move and will simply go extinct.

Wider impact

"This is a really heavy hitting paper because it contributes hard
data to support the long-standing assumption that marine systems have some
of the highest vulnerabilities to climatic warming," says Sarah Diamond,
an ecologist and assistant professor at Case Western Reserve University
in Cleveland, Ohio who did not work on the paper. "This is important
because marine systems can get overlooked."

Most humans are landlubbers, after all—though many of our foods and jobs
are tied to seaborne economies. Pinsky points to species such as Atlantic
halibut, winter flounder, and ocean quahog that have disappeared from historical
habitats and are important to fisheries.

In addition to cutting the greenhouse gas emissions
that are causing climate change, he says that stopping overfishing,
rebuilding overfished populations, and limiting ocean habitat destruction
could help address species loss.

"Setting up networks of marine protected areas that
act as stepping stones as species move to higher latitudes," he adds,
"could help them cope with climate change going forward."

Beyond the sea

The Rutgers study reflects how important it is to measure not just temperature
changes but how they affect animals, says Alex Gunderson,
an assistant professor of ecology and evolutionary biology at Tulane University
in New Orleans who did not work on the study.